What does the phrase "to armor oneself with vows" mean?

This seems to be a common trope-- samnaha. I honestly have no idea what they mean, it strikes me as a "it's raining cats and dogs" sort of idiom. Is there an explanation for what it means? Examples: > For they wear the armor of great vows, accumulate merits, deliver all > beings from birth-and-death, visit Buddha-lands to perform the > bodhisattva practices, make offerings to Buddhas http://www.onmarkproductions.com/html/48-bosatsu-vows.shtml And in Shingon, they visualize themselves putting on armor (or physically don?) Thinking Through Shingon Ritutual "Then one protects the body by donning armor..."